1. Field of the Invention
The present invention relates to image forming apparatuses, and in particular, to an image forming apparatus that feeds a sheet after loosening stacked sheets by blowing air on the sheets.
2. Description of the Related Art
Some image forming apparatuses, such as copiers and printers, each have a sheet feeding device that consecutively separates the topmost sheet from a sheet stack on a sheet stacking unit and feeds the sheet to an image forming unit.
One such sheet feeding device is disclosed in U.S. Pat. No. 5,645,274. The sheet feeding device blows air on a sheet stack on a sheet stacking unit, causes several sheets to be lifted, separates one sheet from the other sheets, and then feeds the sheet while causing the sheet to be held on a suction conveying belt by suction.
FIG. 13 illustrates the structure of such a sheet feeding device, which separates a sheet from a sheet stack by using air and then feeds the sheet while causing the sheet to be held on a suction conveying belt by suction.
In FIG. 13, an image forming apparatus includes a repository 11 for storing sheets S, the repository 11 being slidably movable on a main body (not shown) thereof. The repository 11 is provided with a raisable and lowerable tray 12 for accommodating the plurality of sheets S thereon and a rear-end regulator 13 for regulating the position of an upstream end of the sheets S in a sheet feed direction, i.e., the rear end. The repository 11 is also provided with side-end regulators 14 and 16 and a slide rail 15 used for sliding the repository 11 outward from the main body of the image forming apparatus. The side-end regulators 14 and 16 regulate ends of the sheets S in a width direction, which is substantially perpendicular to the sheet feed direction, i.e., the side ends of the sheets S.
A suction conveying belt 21 is configured to feed a sheet while holding the sheet by suction. A suction fan 36 is configured to cause one of the sheets S to be held on the suction conveying belt 21 by suction. A suction shutter 37 is disposed within a suction duct 38. An air blowing unit 30 is configured to blow air on a downstream (front) end of a sheet stack SA in the sheet feed direction, i.e., the leading-end face. The air blowing unit 30 includes a separation fan 31, a separation duct 32, a loosening nozzle 33, and a separation nozzle 34.
For a sheet feeding device that has the above described structure, after a user draws the repository 11 out, places the sheets S thereon, and returns the repository 11 into the main body, the tray 12 is raised by a driving unit (not shown) in a direction indicated by an arrow A, as illustrated in FIG. 14. The tray 12 stops at a position where the distance between the top surface of the sheet stack SA and the suction conveying belt 21 is B and waits for reception of a feed signal.
In response to an input of a feed signal, the separation fan 31 starts operating and takes in air in a direction indicated by an arrow C, as illustrated in FIG. 15. The air passes through the separation duct 32 and is blown on the leading-end face of the sheet stack SA from the loosening nozzle 33 and the separation nozzle 34 in directions indicated by arrows D and E, respectively. Then, several sheets Sa among the sheet stack SA are lifted, so that the sheet stack SA is loosened. The suction fan 36 starts operating and discharges air in a direction indicated by an arrow F, as illustrated in FIG. 15. At this time, the suction shutter 37 provided in the suction duct 38 is closed.
The side-end regulators 14 and 16 are provided with auxiliary separation fans 17 and 18, respectively. Air from the auxiliary separation fans 17 and 18 is blown on the side ends of the sheet stack SA through openings 14A and 16A. The provision of the auxiliary separation fans 17 and 18 can lift the sheets Sa to loosen the sheet stack more reliably.
When a predetermined period of time has elapsed from the input of the feed signal and the lift of the sheets Sa has become stable, the suction shutter 37 is rotated in a direction indicated by an arrow G, as illustrated in FIG. 16. Thus, the suction fan 36 produces a negative pressure within the suction duct 38, and a sucking force is generated in a direction indicated by an arrow H through suction holes (not shown) of the suction conveying belt 21. Thus, the topmost sheet Sb of the sheets Sa is held on the suction conveying belt 21 by suction.
Lastly, a belt driving roller 41 is rotated in a direction indicated by an arrow J, as illustrated in FIG. 17, thereby feeding the sheet Sb in a direction indicated by an arrow K together with the rotation of the suction conveying belt 21. Subsequently, the sheet Sb is conveyed to a next conveying path by a pair of drawing rollers 42 rotating in directions indicated by arrows M and P.
In such a sheet feeding device (air sheet feeding device), along the subsequent conveying path, a multi-feed detection unit 43 for detecting a multi-feed condition (a situation in which a plurality of stacked sheets are conveyed at a time) is disposed above and below the subsequent conveying path. The multi-feed detection unit 43 includes, for example, two ultrasonic sensors for transmitting and receiving ultrasonic waves. The multi-feed detection unit 43 is configured to detect whether a multi-feed situation is occurring on the basis of a reception level of ultrasonic waves that are irradiated through one or more sheets passing through the conveying path.
Japanese Patent Laid-Open No. 2004-051287 discloses a technique for reliably loosening sheets by, when a multi-feed condition is detected by a multi-feed detection unit, controlling the quantity of air supplied by an air blowing unit to increase temporarily.
Japanese Patent Laid-Open No. 2004-331258 discloses a technique for reliably loosening sheets by counting the number of occurrences of a multi-feed condition and, if that number reaches a predetermined value, changing the setting of the air blowing unit so that the quantity of air is increased.
These known techniques preset the quantity of air blown from the air blowing unit at a plurality of stages and, if a multi-feed condition is detected or if the number of occurrences of a multi-feed condition reaches a predetermined value, controls the quantity of air to be increased by one stage. In addition, the techniques control the quantity of air to return to the initial setting when one job is completed.
Unfortunately, for these structures, when a multi-feed condition is caused by rotation of a fan for blowing air with an inappropriate rotation frequency, the quantity of air is simply increased in stages. Therefore, the quantity of air does not become appropriate in most cases, so a multi-feed problem is not fully solved. In addition, since the quantity of air returns to the initial setting upon completion of a job, a multi-feed condition tends to occur again when a next job that is to convey the same kind of sheets begins.